1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus employing, for example, a magnetic disk or a magnetooptical disk, and more particularly to a velocity control device for an information recording and/or reproducing head for access to a target track, adapted for use in such apparatus.
2. Related Background Art
For velocity control of such a head (or a light beam spot in the case of an optical disk), there has generally been employed a closed loop control in which, as shown in FIG. 1, generated is a reference velocity proportional to the square root of the residual distance to the target position and the head is made to follow said reference velocity. A control method, the actual head velocity is detected by a velocity detecting circuit, and the control for following said reference velocity is achieved by the thus detected speed signal.
FIG. 2 shows an example of said velocity detecting circuit, wherein provided are a shaping circuit 100 for shaping a tracking error signal into a binary signal; a digital differentiating circuit 101 composed of a delay circuit 102 and an exclusive OR circuit 103; and a monostable multivibrator 104.
Said velocity detecting circuit functions in the following manner. At first, as shown in FIG. 12, an automatic tracking (AT) error signal is binary digitized by the shaping circuit 100 into a pulse signal with a duration corresponding to a half cycle of said error signal. Then the digital differentiating circuit 101 generates trigger pulses, for supply to the monostable multivibrator 104, at the upshift and downshift of said binary signal. In response, the monostable multivibrator 104 generates a pulsed velocity signal with a duration .tau., which varies the cycle time thereof according to the change in cycle time of the AT error signal. Since the velocity is proportional to the cycle time of the AT error signal, the velocity signal can be generated by the processing of said AT error signal AT. Also, said digital velocity signal can be converted, by averaging, into an analog signal.
However, the output of said velocity detecting circuit cannot be practically used as the velocity signal when the velocity approaches zero, as the interval of output pulses of the monostable multivibrator is extended. Consequently an exact closed loop control is difficult in the speed control method explained in FIG. 1. In addition, in the above-explained control method, the band region of the closed loop is limited by the resonance, usually in the 2 to 3 kHz region, of the actuator mechanism for head driving. For this reason the response is deteriorated in a rapid deceleration, whereby the velocity at the end of a seeking operation cannot be brought to zero, and overrun of the target position results. In order to avoid this drawback, there is employed a method of applying a large current to the actuator achieving acceleration within a short time, and effecting the deceleration in a gradual manner. However, such a control method results in an increased seek time because the deceleration is only gradually achieved without fully utilizing the accelerating ability.
In order to avoid such a drawback there is proposed a control method shown in FIG. 4, in which an open-loop bang-bang control is used for a major portion of the access distance, and the closed-loop control is adopted from immediately before the target position. This control method can reduce the seek time, because rapid deceleration is possible with a large current.
However, this control method may result in an overrun or leave a residual distance to the target position if the acceleration or deceleration is not conducted at appropriate points due to the influence of friction, external force, etc. since the bang-bang control is conducted in an open loop. On the other hand, the aforementioned closed-loop control method has to sacrifice the seek time in order to prevent the overrun. Thus, it is difficult to achieve high speed and accuracy at the same time in either method.